1. Field of the Invention
The present invention relates to a polishing method and a polishing apparatus for polishing a substrate, such as a wafer, and more particularly to a polishing method and a polishing apparatus that change a polishing condition in accordance with an elastic modulus of a polishing pad which is used in polishing the substrate.
2. Description of the Related Art
A CMP (chemical mechanical polishing) apparatus polishes a surface of a wafer by providing sliding contact between the wafer and a polishing pad in the presence of a polishing liquid, while pressing the wafer against the polishing pad. The polishing pad is formed from an elastic material, such as porous polyurethane. A top surface of the polishing pad provides a polishing surface for polishing the wafer, which is placed in sliding contact with this polishing surface.
The polishing surface of the polishing pad is regularly processed by a pad dresser (or a pad conditioner). This pad dresser has a dressing surface having abrasive gains, such as diamond particles, fixed thereto. The pad dresser presses this dressing surface against the polishing pad while rotating the dressing surface to scrape away the surface of the polishing pad slightly to thereby restore the polishing surface. As the dressing process (or the conditioning process) is repeated, the polishing pad becomes thinner gradually. Further, as the polishing of the wafer is repeated, the polishing liquid gradually soaks into cells formed in the polishing pad. As a result, an elastic modulus of the polishing pad changes.
The elastic modulus of the polishing pad is a value of physical property representing a difficulty of being deformed when a force is applied to the polishing pad. Specifically, a higher elastic modulus indicates a harder polishing pad. The elastic modulus of the polishing pad depends not only on a thickness of the polishing pad and the existence of the polishing liquid that has soaked into the polishing pad, but also on a temperature of the polishing pad. Typically, the polishing pad is made of resin as described above. Therefore, as the temperature of the polishing pad increases, the polishing pad becomes soft.
The elastic modulus of the polishing pad greatly affects a polishing profile of the wafer. In particular, when the polishing pad is soft, the wafer, which is pressed against the polishing pad, sinks into the polishing pad. As a result, a peripheral portion of the wafer is excessively polished as compared with other portions of the wafer. This is a so-called rounded edge. In order to prevent such an undesired polishing result, it is preferable to change wafer polishing conditions based on the elastic modulus of the polishing pad.
In a conventional technique, the elastic modulus of the polishing pad is measured so that a remaining lifetime of the polishing pad is determined or conditions of the dressing process are adjusted based on the elastic modulus (see U.S. patent document US 2006/0196283). However, the conventional technique does not provide the use of the measured elastic modulus for adjusting the polishing conditions of the wafer.
It has been proposed to measure the temperature of the polishing pad and to estimate the elastic modulus of the polishing pad from the measured pad temperature (for example see Japanese laid-open patent publication No. 2012-148376). However, the elastic modulus of the polishing pad depends not only on the temperature of the polishing pad, but also on other factors as described above. Therefore, the estimated elastic modulus of the polishing pad can be different from an actual elastic modulus.